A series of nitro 5-deazaflavins, 5-deazaflavins possessing a nitro group at C(6)-C(9) position, has been designed and synthesized as a novel class of bioreductive nitrohetero-aromatic compounds and their cytotoxicities towards LI210 and KB cells were evaluated. It has been found that the nitro 5-deazaflavins undergo one electron reduction on the nitro group and undergo two electrons or "(net) hydride" reduction on the C(5)-C(4a)-C(10a)-N(1) redox system. They showed much more potent antitumor activities than the other 5-deazaflavins bearing no nitro group. These results suggest that an activation of nitro group by biological one electron reduction is crucial for an expression of cytotoxicity.